The PDPs can be implemented as high-resolution and large-size displays, and therefore, have been commercialized as television sets having a diagonal screen of 65 inches or large-scale displays in public places, and are even planned to be commercialized as the devices having a diagonal screen of over 100 inches. In particular, the PDPs used for TV sets are going to be applied to the Full High Definition TVs, which have twice or more number of scanning lines than the conventional NTSC TVs.
The PDP is basically composed of a front panel and a rear panel. The front panel is composed of a glass substrate made of float-processed sodium borosilicate glass. Display electrodes composed of transparent electrodes and bus electrodes having a stripe form are disposed on one major surface of the glass substrate. A dielectric layer covering the display electrodes is formed so as to function as a capacitor. A protecting layer made of magnesium oxide (MgO) is formed on the dielectric layer. On the other hand, the rear panel is composed of another glass substrate provided with a fine hole for exhausting an air and filling (also referred to as “introducing”) a discharge gas. On one major surface of the another glass substrate, address electrodes (also referred to as “data electrodes”) are arranged in a stripe form. An underlying dielectric layer is formed so as to cover the address electrodes. Barrier ribs are formed on the underlying dielectric layer. Phosphor layers for respectively generating visible light of red, green and blue colors are formed between the adjacent barrier ribs.
The front and rear panels are placed so that the major surfaces thereof provided with the electrodes thereon face each other. The peripherals of the front panel and the rear panel, and an exhaust pipe for exhausting an air and filling a discharge gas are sealed in airtight with adhesives. The discharge space divided by the barrier ribs is once exhausted and then filled with the discharge gas (at a pressure of 400 Torr to 600 Torr for Ne—Xe gas) via the exhaust pipe, and thereafter, sealed in airtight by locally heating and melting (i.e., chipping off) an adequate portion of the exhaust pipe. An image signal voltage is selectively applied to the display electrodes in the completed PDP to thereby cause discharge that generates ultraviolet light. The ultraviolet light excites respective phosphor layers to generate visible light of red, green and blue colors. Thus, the PDP implements a color image display.
Low-melting glass (also referred to as “frit glass”), which generally contains lead oxide as a major constituent, is used as the adhesive that is used for sealing the dielectric layer, the peripherals of the front and rear panels, and the exhaust pipe in the above-stated PDP. The frit glass includes amorphous frit glass and crystalline frit glass. The amorphous one is not crystallized but keeps amorphous characteristics even when heated, whereas the crystalline one is crystallized when heated. Those two materials each have both advantages and disadvantages, and therefore, are selected for use in consideration of suitability for the manufacturing steps. A pasty adhesive formed by kneading the mixture of the frit glass and filler with organic solvent is used to seal the peripheries of the front and rear panels. First, the adhesive is placed at a specified position in the periphery of at least one of the front and rear panels by use of a thick film printer or a coater provided with an ink jet or a dispenser. Next, preliminary firing is performed at such a specified temperature in advance that the frit glass is not melted (or not softened), before the front and rear panels are positioned to face each other and assembled.
To seal the exhaust pipe, material formed by kneading the mixture of the frit glass and filler with organic solvent is first prepared, like in sealing the peripheries of the front and rear panels. Next, the material is molded into a form having a through-hole 233 at the center portion thereof by use of a mold, as shown in the perspective view of FIG. 5. The molded material is then fired at such a temperature that the solvent is evaporated; thereby a sintered adhesive referred to as tablet 232 is obtained. Tablet 232 is used for sealing the exhaust pipe.
Environmental concerns of recent years require non-lead material, which contains no lead and is called “lead-free” or “lead-less”, to be used also for the PDP. As examples of the adhesive, a lead-free phosphate (such as phosphate-tin oxide) adhesive and a bismuth oxide adhesive are disclosed (e.g., in Patent Document 1, and Patent Document 2). The adhesive that contains, as a major constituent, phosphate-tin oxide low-melting glass, which has been proposed as the non-lead adhesive, is lower in water resistance than a lead oxide adhesive, which has conventionally been used. This results in a problem that the PDP hardly maintains sufficiently high airtightness, and therefore, a bismuth oxide adhesive is noticed as non-lead material. The non-lead adhesive that contains bismuth oxide frit glass as a major constituent is characterized by crystallization during firing, and is known to incline higher in softening point than lead-containing ordinary amorphous frit glass. Lead-containing borosilicate glass has been used for the conventional exhaust pipe due to its relatively low softening point and high working efficiency at the sealing step, whereas it is now being replaced with non-lead borosilicate glass because of environmental consideration.
FIGS. 6A and 6B are each a schematic cross-sectional view showing a procedure of sealing the peripheries of front panel 222 and rear panel 223 and exhaust pipe 221 with adhesives 231a and 231b. With reference to FIG. 6A, first, adhesive 231a or 231b is applied to a specified portion of the periphery of at least one of front panel 222 and rear panel 223 by use of a thick film printer or a coater provided with an ink jet or a dispenser. Front panel 222 and rear panel 223 are positioned on each other so that the display electrodes on front panel 222 and data electrodes on rear panel 223 intersect at a right angle and are aligned at a specified position with each other. Front panel 222 and rear panel 223 are then fixed with each other with a fastening tool (not shown). Next, tablet 232 is mounted on rear panel 223 so that the center of through hole 233 formed at the center portion of tablet 232 coincides with the center of fine exhaust hole 230 disposed at a specified position around the corner of rear panel 223. Exhaust pipe 221 is positioned on tablet 232 so that the center of one open end of exhaust pipe 221 substantially coincides with the center of fine exhaust hole 230, and is assembled and fixed with rear panel 223 and tablet 232 with another fastening tool (not shown) so as to prevent the centers from being displaced. Finally, the respective adhesives, which are applied to front panel 222, rear panel 223 and exhaust pipe 221 assembled and fixed with each other with the fastening tools, are heated to melt, and thereafter are cooled down to solidify. Thus, the sealing process has been performed.
FIG. 6A shows an arrangement where the open end of exhaust pipe 221 to be sealed is at a lower side and the other open end thereof to be connected with an exhaust device is at a higher side. In this arrangement, a connecting pipe for connecting exhaust pipe 221 with the exhaust device has to be long and bent. Therefore, another arrangement, where exhaust pipe 221 is placed up side down from that shown in FIG. 6A, has been used in some cases. Specifically, the arrangement shown in FIG. 6B, where the open end of exhaust pipe 221 to be connected with the exhaust device faces down, is used for the sealing process in some cases. The arrangement shown in FIG. 6B allows exhaust pipe 221 to easily and directly be connected with the exhaust device, and therefore, allows exhaust pipe 221 to be shortened and an exhaust period to be reduced advantageously.
The sealing process performed by using the arrangement shown in FIG. 6B, where the open end of the exhaust pipe to be connected with the exhaust device faces down, well prevents the softened (or melted) adhesive from dripping, if tablet 232 is formed of an adhesive mainly made of lead-containing frit glass.
However, the sealing process performed by using the same arrangement, sometimes causes dripping of the adhesive during the sealing step, if the tablet is formed of an adhesive mainly made of non-lead frit glass. The dripping of the adhesive deteriorates airtightness at the sealed portion of the exhaust pipe, and has thereby prevented a reliable sealing.
Patent Document 1: Unexamined Japanese Patent Publication No. 2004-182584,
Patent Document 2: Unexamined Japanese Patent Publication No. 2003-095697